Lichig0

The reason is more like: if you don't know the mechanics of frameworks you're stuck with them. Even though they can make things easier in some instances they come with drawbacks – complexity, file sizes, more points of error, longer load times and at some points even limitations of your possibilities at hand. You better know how the stuff is working under the hood so you can make an educated decision wether it's a good idea to use a framework or not. @DarshanDash think of things like grids on websites. Instead of calculating everything by hand and adjusting it with your very own Javascript or CSS snippets you can use a frameworks that just does the job for you. 
 
Most frameworks aren't written in just one language. Bootstrap uses a lot of Javascript for instance which can make a website load slower. There are other frameworks around, some that do avoid Javascript entirely (i.e. Bourbon in pure SASS, a precompiler for CSS adding several "missing" functionalities to CSS). There are frameworks basically every single application you can think of in almost every single language there is.
 
@DarshanDash you should really first learn the very basics of CSS and HTML before jumping into using frameworks. You need to learn what those frameworks are doing in order to use them properly and don't make a mess out of a project. 

Locked.
 
We won't allow this type of discussion.

Well, you're looking at a variety of JS frameworks/libraries. The most popular ones are Ract (made by Facebook), AngularJS (made by Google) and Vue.js (independent). React is probably the most popular one right now. You have to differentiate between the logic of a dynamic UI and the components. There are countless component libraries for all of those frameworks/libraries. If you know your modern (!!!) Javascript, you should jst look into them and their different concepts. Personally I like Ract the most and don't get that well along with AngularJS. It's a matter of taste. Here is a random list of 11 component libraries for React (no affiliation with that website): https://blog.bitsrc.io/11-react-component-libraries-you-should-know-178eb1dd6aa4
 
Codecademy has a nice introduction into React.
 
Basically: if you want dynamic web frontends you have to go the Javascript way. Responsiveness on the other hand is very much possible with pure CSS3 and HTML5.
 
All depends on the frameworks you choose. Remember that you have to handle your routes in the backend as well. Your command line tool on its own won't cut the deal. You need a web framework that handles the routes and if neccessary interacts with the webserver and the queries. For Python there are Flask and Django (next to a few quite small frameworks). You could also level up your Javascript game and go the node.js route and let your node.js interface interact with your comannd line tool written in python. 

As Lichig0 said. You have to be careful with Javascript as it does lead to bad practices if you want to evolve to any other languages.

If you're learning C# already, Java might not be a bad place to start. I've never worked or learned C# but I've heard and read that it's quite similar to Java.
 
While yes, JS is easy to pick up, I personally think it can lead to bad practices and a misguided sense of reasoning how things work.